Electropolishing platinum in a molten bath of potassium thiocyanate and potassium cyanide



United States Patent 3 419,485 ELECTROPOLISHING PLATINUM IN A MOLTEN BATH OF POTASSIUM THIOCYANATE AND P0- TASSIUM CYANIDE Robert Seegmiller and James K. Gore, Los, Alamos, N. Me'x., assignors to the United States of America as represented by the United States Atomic Energy Commission a No Drawing. Filed June 3, 1966, Ser. No. 555,931

' 4 Claims. (Cl. 204-1405) The invention described herein was made in the course of, or under, Contract W-7405-ENG-36 with the US. Atomic Energy Commission.

This invention is concerned with a method of electropolishing platinum, and in particular to a rnethod in which the platinum is electricolytically treated in a molten solution containing potassium cyanide in potassium thiocyanate.

Platinum is being used for many purposes such as the orifice in a cell separation apparatus that requires small, and/or complex shapes and where mechanical polishing is impractical or undesirable. In this connection there was needed a process to round, smooth, and polish platinum surfaces of these small, and/or complex shapes. The inventors have discovered a method that is simple, direct, and inexpensive to achieve the said solution to this prob lem. In particular, this invention is directed to electropolisbing platinum to a brilliant finish by anodic treat ment in a molten solution of potassium cyanide in potassium thiocyanate.

It is, therefore, an object of this invention to provide a process for electropolishing, smoothing, or rounding a platinum surface.

Other objects of this invention will become apparent from the following description of the preferred embodiment.

The platinum metal is placed in an electrolytic bath containing molten potassium cyanide in potassium thiocyanate. The best proportions are 1 part potassium cyanide to 10 parts potassium thiocyanate, although the ratio is not critical. The optimum temperature of the solution is about 300 C. with a permissible operating range of 200-300 C. The current density range for polishing is 1 to 5 amps per square inch of metal with a current density of 2.5 amps per square inch being the optimum.

The platinum metal is connected to a suitable power source, said metal acting as the anode while a tantalum metal cathode has proved to be most satisfactory. The electrolytic bath is contained in a high temperature glass 3,419,485 Patented Dec. 31, 1968 receptacle of suitable dimensions. Best polishing action is obtained on platinumsurfaces facing upward and sufliciently inclined from the vertical that the gas bubbles rise without sweeping along the surface.

The platinum apparently dissolves in the +2 state with nearly current efiiciency, giving a polishing rate of about 0.3 mils/minute at 2 amp/sq. in. The bath decomposes fairly rapidly, with about 2 working days at temperature, the longest useful bath life. The appearance of a salty looking scum on the top of the bath signals the end of the useful life of the bath.

Because of the extremely corrosive nature of this solution, suitable materials for cathodes andconnections to the piece to be polished are limited. The inventors have found that tantalum wire resisted anodic attack in the solution and is useful for suspending the articles to be electropolished.

Rhodium cathodes are also satisfactory although they are etched to a significant degree. Nickel cathodes are also usable, but they darkened the solution. to the point that visual observation of the polishing action is impossible.

What is claimed is:

1. A method of electropolishing platinum to a brilliant mirror finish comprising electrically biasing platinum anodically in amolten solution consisting essentially of potassium cyanide and potassium thiocyanate, said solution being at a temperature in the range of 200-320 C. and the current density being between 1-5 amp/sq. In.

2. The method of claim 1 in which the current density is about 2.5 amps/sq. in.- Y

3. The method of claim 1 in which the temperature of the solution is about 300 C.

4. The method of claim 1 in which the ratio by weight of potassium cyanide to potassium thiocyanate is 1:10.

References Cited UNITED STATES PATENTS 9/1937 Hall 204-39 3/ 1967 Andrews et a1. 204-39 OTHER REFERENCES ROBERT K. MIHALEK, Primary Examiner. 

1. A METHOD OF ELECTROPOLISHING PLATINUM TO A BRILLIANT MIRROR FINISH COMPRISING ELECTRICALLY BIASING PLATINUM ANODICALLY IN A MOLTEN SOLUTION CONSISTING ESSENTIALLY OF POTASSIUM CYANIDE AND POTASSIUM THIOCYANATE, SAID SOLUTION BEING AT A TEMPERATURE IN THE RANGE OF 200*-320* C. AND THE CURRENT DENSITY BEING BETWEEN 1-5 AMP/SQ. IN. 